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<tr class="t-nv-h1"><td colspan="5"> 类型 </td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="streamoff.html" title="cpp/io/streamoff"><span class="t-lines"><span>streamoff</span></span></a></div></div></td></tr>
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<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><strong class="selflink"><span class="t-lines"><span>fpos</span></span></strong></div></div></td></tr>
<tr class="t-nv-h1"><td colspan="5"> 错误类别接口 </td></tr>
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<table class="t-dcl-begin"><tbody>
<tr class="t-dsc-header">
<td> <div>定义于头文件 <code><a href="../header/ios.html" title="cpp/header/ios">&lt;ios&gt;</a></code>
 </div></td>
<td></td>
<td></td>
</tr>
<tr class="t-dcl">
<td class="t-dcl-nopad"> <div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1">&lt;</span> <span class="kw1">class</span> State <span class="sy1">&gt;</span><br>
<span class="kw1">class</span> fpos<span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad">  </td>
<td class="t-dcl-nopad">  </td>
</tr>
<tr class="t-dcl-sep"><td></td><td></td><td></td></tr>
</tbody></table>
<p>类模板 <span class="t-lc"><strong class="selflink">std::fpos</strong></span> 的特化标识流或文件中的绝对位置。每个 <code>fpos</code> 类型对象保有流中的字节位置（常为 <span class="t-lc"><a href="streamoff.html" title="cpp/io/streamoff">std::streamoff</a></span> 类型的私有成员）和当前迁移状态， <code>State</code> 类型值（常为  <span class="t-lc"><a href="../string/multibyte/mbstate_t.html" title="cpp/string/multibyte/mbstate t">std::mbstate_t</a></span> ）。
</p><p>提供下列 <span class="t-lc"><strong class="selflink">std::fpos</strong></span> 特化：
</p>
<table class="t-dsc-begin">

<tr class="t-dsc-sep">
<td colspan="2">  
</td></tr>
<tr class="t-dsc-hitem">
<td> 类型
</td>
<td> 定义
</td></tr>


<tr class="t-dsc">
<td>  <code>streampos</code>
</td>
<td>  <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">fpos</span><span class="sy1">&lt;</span><a href="../string/char_traits.html"><span class="kw1267">std::<span class="me2">char_traits</span></span></a><span class="sy1">&lt;</span><span class="kw4">char</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">state_type</span><span class="sy1">&gt;</span></span></span>
</td></tr>

<tr class="t-dsc">
<td>  <code>wstreampos</code>
</td>
<td>  <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">fpos</span><span class="sy1">&lt;</span><a href="../string/char_traits.html"><span class="kw1267">std::<span class="me2">char_traits</span></span></a><span class="sy1">&lt;</span><span class="kw4">wchar_t</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">state_type</span><span class="sy1">&gt;</span></span></span>
</td></tr>

<tr class="t-dsc">
<td>  <code>u8streampos</code> <span class="t-mark-rev t-since-cxx20">(C++20 起)</span>
</td>
<td>  <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">fpos</span><span class="sy1">&lt;</span><a href="../string/char_traits.html"><span class="kw1267">std::<span class="me2">char_traits</span></span></a><span class="sy1">&lt;</span>char8_t<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">state_type</span><span class="sy1">&gt;</span></span></span>
</td></tr>

<tr class="t-dsc">
<td>  <code>u16streampos</code> <span class="t-mark-rev t-since-cxx11">(C++11 起)</span>
</td>
<td>  <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">fpos</span><span class="sy1">&lt;</span><a href="../string/char_traits.html"><span class="kw1267">std::<span class="me2">char_traits</span></span></a><span class="sy1">&lt;</span><span class="kw4">char16_t</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">state_type</span><span class="sy1">&gt;</span></span></span>
</td></tr>

<tr class="t-dsc">
<td>  <code>u32streampos</code> <span class="t-mark-rev t-since-cxx11">(C++11 起)</span>
</td>
<td>  <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">fpos</span><span class="sy1">&lt;</span><a href="../string/char_traits.html"><span class="kw1267">std::<span class="me2">char_traits</span></span></a><span class="sy1">&lt;</span><span class="kw4">char32_t</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">state_type</span><span class="sy1">&gt;</span></span></span>
</td></tr>
</table>
<p><code>fpos</code> 的所有特化都符合<a href="../named_req/DefaultConstructible.html" title="cpp/named req/DefaultConstructible"><i>可默认构造</i> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(DefaultConstructible)</span> </a>、<a href="../named_req/CopyConstructible.html" title="cpp/named req/CopyConstructible"><i>可复制构造</i> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(CopyConstructible)</span> </a>、<a href="../named_req/CopyAssignable.html" title="cpp/named req/CopyAssignable"><i>可复制赋值</i> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(CopyAssignable)</span> </a>、<a href="../named_req/Destructible.html" title="cpp/named req/Destructible"><i>可析构</i> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Destructible)</span> </a>和<a href="../named_req/EqualityComparable.html" title="cpp/named req/EqualityComparable"><i>可相等比较</i> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(EqualityComparable)</span> </a>要求。
</p><p>若 <code>State</code> 可平凡复制构造，则 <code>fpos</code> 拥有平凡默认构造函数。
若 <code>State</code> 可平凡复制赋值，则 <code>fpos</code> 拥有平凡复制赋值运算符。
若 <code>State</code> 可平凡析构，则 <code>fpos</code> 拥有平凡析构函数。
</p>
<h3><span class="mw-headline" id=".E6.A8.A1.E6.9D.BF.E5.BD.A2.E5.8F.82">模板形参</span></h3>
<table class="t-par-begin">


<tr class="t-par">
<td>  State
</td>
<td> -
</td>
<td> 表示迁移状态的类型
</td></tr>
<tr class="t-par-hitem">
<td colspan="3"> 类型要求
</td></tr>
<tr class="t-par-req">
<td colspan="3"> -<div class="spacer"></div><code>State</code> 必须满足<a href="../named_req/Destructible.html" title="cpp/named req/Destructible"><i>可析构</i> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Destructible)</span> </a> 、 <a href="../named_req/CopyAssignable.html" title="cpp/named req/CopyAssignable"><i>可复制赋值</i> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(CopyAssignable)</span> </a> 、 <a href="../named_req/CopyConstructible.html" title="cpp/named req/CopyConstructible"><i>可复制构造</i> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(CopyConstructible)</span> </a> 和 <a href="../named_req/DefaultConstructible.html" title="cpp/named req/DefaultConstructible"><i>可默认构造</i> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(DefaultConstructible)</span> </a> 的要求。
</td></tr></table>
<h3><span class="mw-headline" id=".E6.88.90.E5.91.98.E5.87.BD.E6.95.B0">成员函数</span></h3>
<table class="t-dsc-begin">

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="fpos/state.html" title="cpp/io/fpos/state"> <span class="t-lines"><span>state</span></span></a></div></div>
</td>
<td>  获取/设置迁移状态的值 <br> <span class="t-mark">(公开成员函数)</span>
</td></tr>
</table>
<p>另外，提供支持下列操作的成员和非成员函数：
</p>
<ul><li> 默认构造函数，存储零偏移并值初始化状态对象。
</li></ul>
<ul><li> 非 explicit 的构造函数，接受一个（可为 const 的） <span class="t-lc"><a href="streamoff.html" title="cpp/io/streamoff">std::streamoff</a></span> 类型参数，存储其偏移并值初始化状态对象。此构造函数必须亦接受特殊值 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="streamoff.html"><span class="kw1803">std::<span class="me2">streamoff</span></span></a><span class="br0">(</span><span class="sy2">-</span><span class="nu0">1</span><span class="br0">)</span></span></span> ：某些流操作返回以此方式构造的 <code>std::fpos</code> ，以指示错误。
</li></ul>
<ul><li> 从（可为 const 的） <code>fpos</code> 到 <span class="t-lc"><a href="streamoff.html" title="cpp/io/streamoff">std::streamoff</a></span> 的显式转换。结果是存储的偏移。
</li></ul>
<ul><li> 比较二个（可为 const 的） <code>std::fpos</code> 类型，返回可转换为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span></span></span> 的类型的 <span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="sy1">==</span></span></span> 和 <span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="sy3">!</span><span class="sy1">=</span></span></span> 。 <span class="t-c"><span class="mw-geshi cpp source-cpp">p <span class="sy3">!</span><span class="sy1">=</span> q</span></span> 等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">!</span><span class="br0">(</span>p <span class="sy1">==</span> q<span class="br0">)</span></span></span> 。
</li></ul>
<ul><li> <span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="sy2">+</span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="sy2">-</span></span></span> ，使得对于（可为 const 的） <code>fpos&lt;State&gt;</code> 对象 <code>p</code> 和（可为 const 的） <span class="t-lc"><a href="streamoff.html" title="cpp/io/streamoff">std::streamoff</a></span> 对象 <code>o</code>
</li></ul>
<dl><dd><ul><li> <span class="t-c"><span class="mw-geshi cpp source-cpp">p <span class="sy2">+</span> o</span></span> 拥有 <code>fpos&lt;State&gt;</code> 类型，而存储的偏移为将 <code>o</code> 加到 <code>p</code> 的结果
</li><li> <span class="t-c"><span class="mw-geshi cpp source-cpp">o <span class="sy2">+</span> p</span></span> 拥有可转换为 <code>fpos&lt;State&gt;</code> 的类型，而转换结果等于 <span class="t-c"><span class="mw-geshi cpp source-cpp">p <span class="sy2">+</span> o</span></span>
</li><li> <span class="t-c"><span class="mw-geshi cpp source-cpp">p <span class="sy2">-</span> o</span></span> 拥有 <code>fpos&lt;State&gt;</code> 类型，而存储的偏移为从 <code>p</code> 的偏移减去 <code>o</code> 的结果
</li></ul>
</dd></dl>
<ul><li> <span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="sy2">+</span><span class="sy1">=</span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="sy2">-</span><span class="sy1">=</span></span></span> ，能接受一个（可为 const 的） <span class="t-lc"><a href="streamoff.html" title="cpp/io/streamoff">std::streamoff</a></span> 并分别对存储的偏移加/减之。
</li></ul>
<ul><li> <span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="sy2">-</span></span></span> ，能对二个（可为 const 的） <code>std::fpos</code> 对象相减，产生 <span class="t-lc"><a href="streamoff.html" title="cpp/io/streamoff">std::streamoff</a></span> ，使得对于二个这种对象 <code>p</code> 与 <code>q</code> 有 <span class="t-c"><span class="mw-geshi cpp source-cpp">p <span class="sy1">==</span> q <span class="sy2">+</span> <span class="br0">(</span>p <span class="sy2">-</span> q<span class="br0">)</span></span></span> 。
</li></ul>
<h3><span class="mw-headline" id=".E6.B3.A8.E6.84.8F">注意</span></h3>
<p>要求 <code>std::streampos</code> 与 <code>std::wstreampos</code> 为同一类型，因为要求 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../string/char_traits.html"><span class="kw1267">std::<span class="me2">char_traits</span></span></a><span class="sy1">&lt;</span><span class="kw4">char</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">state_type</span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../string/char_traits.html"><span class="kw1267">std::<span class="me2">char_traits</span></span></a><span class="sy1">&lt;</span><span class="kw4">wchar_t</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">state_type</span></span></span> 均为 <span class="t-lc"><a href="../string/multibyte/mbstate_t.html" title="cpp/string/multibyte/mbstate t">std::mbstate_t</a></span> 。 C++98 有过自相矛盾的陈述，说若实现支持窄面向 iostream 中的无迁移编码，则它们可以不同，但在宽面向流中支持一或多个迁移编码，不过这在 C++03 中更正了。
</p><p>一些 <a href="../io.html" title="cpp/io">I/O 流成员函数</a>返回并操纵成员 typedef <code>pos_type</code> 类型对象。对于流，这些成员 typedef 由模板形参 <code>Traits</code> 提供，默认为 <span class="t-lc"><a href="../string/char_traits.html" title="cpp/string/char traits">std::char_traits</a></span> ，它定义其 <code>pos_type</code> 为 <code>std::fpos</code> 的特化。 <span class="t-c"><span class="mw-geshi cpp source-cpp">Traits<span class="sy4">::</span><span class="me2">pos_type</span></span></span> 不是 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">fpos</span><span class="sy1">&lt;</span><a href="../string/multibyte/mbstate_t.html"><span class="kw1179">std::<span class="me2">mbstate_t</span></span></a><span class="sy1">&gt;</span></span></span> （ <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">streampos</span></span></span> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">wstreampos</span></span></span> 的别名）时， I/O 流库的行为是实现定义的。
</p>
<h3><span class="mw-headline" id=".E7.BC.BA.E9.99.B7.E6.8A.A5.E5.91.8A">缺陷报告</span></h3>
<p>下列更改行为的缺陷报告追溯地应用于以前出版的 C++ 标准。
</p>
<table class="dsctable" style="font-size:0.8em">
<tr>
<th> DR
</th>
<th> 应用于
</th>
<th> 出版时的行为
</th>
<th> 正确行为
</th></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://wg21.link/P0759R1">P0759R1</a>
</td>
<td> C++98
</td>
<td> 规定不清晰且不完整
</td>
<td> 已整理
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://wg21.link/P1148R0">P1148R0</a>
</td>
<td> C++11
</td>
<td> 不明确哪个头文件定义 <code>u16streampos</code> 和 <code>u32streampos</code> 或其定义是什么
</td>
<td> 使之明确
</td></tr></table>
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
<table class="t-dsc-begin">

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="streamoff.html" title="cpp/io/streamoff"> <span class="t-lines"><span>streamoff</span></span></a></div></div>
</td>
<td>  表示相对的文件/流位置（距 fpos 的偏移），足以表示任何文件大小 <br> <span class="t-mark">(typedef)</span> </td></tr>

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="basic_ostream/tellp.html" title="cpp/io/basic ostream/tellp"> <span class="t-lines"><span>tellp</span></span></a></div></div>
</td>
<td>  返回输出位置指示器 <br> <span class="t-mark">(<code>std::basic_ostream&lt;CharT,Traits&gt;</code> 的公开成员函数)</span> </td></tr>

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="basic_ostream/seekp.html" title="cpp/io/basic ostream/seekp"> <span class="t-lines"><span>seekp</span></span></a></div></div>
</td>
<td>  设置输出位置指示器 <br> <span class="t-mark">(<code>std::basic_ostream&lt;CharT,Traits&gt;</code> 的公开成员函数)</span> </td></tr>

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="c/fgetpos.html" title="cpp/io/c/fgetpos"> <span class="t-lines"><span>fgetpos</span></span></a></div></div>
</td>
<td>  获取文件位置指示器 <br> <span class="t-mark">(函数)</span> </td></tr>
</table>

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